See my email to mailing list. This will slightly break anyone who upgrades while running and expects to see their prompt history, and leave a stray file, I think, but nothing else, and it'll permanently improve tab-completion, and is tidier.
XMonad.Prompt.Input is a new module which provides a framework for
prompting the user for input and passing it along to some other action,
useful for building actions which require user input.
XMonad.Prompt.Email is a simple example of the use of XMonad.Prompt.Input,
which prompts the user for a recipient, subject, and body, and sends
a one-line email.
I also made a small refactoring to XMonad.Prompt in order to support
XMonad.Prompt.Input.